A method for controlling torque boost of a switched reluctance motor without position sensor

Abstract

The present invention proposes a position sensorless torque boost control method for a switched reluctance motor based on phase current slope, and belongs to the technical field of switched reluctance motor control. In the voltage chopping control mode, the maximum point of the phase current, i.e., the position where the stator and rotor begin to align, can be detected by detecting the negative zero crossing point of the phase current slope, and then the rotor position information can be estimated according to this special position point. However, in this case, the voltage pulse is applied at a constant duty cycle during the entire conduction period, the DC bus voltage is not fully utilized, and the motion back electromotive force is significantly greater than the average voltage applied to both ends of the winding, and the phase current is significantly suppressed, resulting in a reduction in output torque. In view of this problem, a position sensorless torque boost control method for a switched reluctance motor based on phase current slope is proposed. After detecting the maximum point of the phase current, the proposed method switches the control mode from voltage chopping control to current chopping control, thereby making full use of the available DC bus voltage, and can adjust the current, thereby obtaining a higher output torque. The method estimates the rotor position information without the need to predict the electromagnetic characteristic data and body parameters of the motor, is simple to implement, has low calculation requirements, and can improve the output torque of the motor, and has strong practicality.

Description

Technical Field

[0001] The invention relates to a position sensorless torque boost control method for a switched reluctance motor, belonging to the technical field of switched reluctance motor control. Background Art

[0002] The switched reluctance motor adopts a double salient pole structure, with a concentrated winding on the stator and no winding and permanent magnet on the rotor. It has the characteristics of simple and strong structure, strong fault tolerance, high operating efficiency, etc., and is suitable for high-speed operation and harsh working environment. At the same time, the strong fault tolerance of the switched reluctance motor has also broadened its application field. It has good application prospects in many occasions such as aerospace, electric vehicles, ships, distributed power systems, etc., especially in some applications such as aircraft engines and nuclear power plants that need to work under harsh conditions such as high temperature, high pollution and strong radiation. It has unique application advantages. In high-performance control applications, the rotor position information must be accurately detected. The traditional method is to install a position sensor. However, the position sensor is particularly susceptible to interference from ambient temperature, humidity, and electromagnetic interference, which reduces the reliability of the speed control system. Therefore, it is of great significance to study the position sensorless technology of the switched reluctance motor.

[0003] In the past three decades, a variety of sensorless position estimation methods such as high-frequency pulse injection, modulation and mediation, current / flux method, inductance model method, intelligent control method and observer method have been widely studied. These position estimation methods have their own operating conditions and scope of application, and each has its own advantages and disadvantages. When running at medium and high speeds, the current / flux method, inductance model method and intelligent control method are widely used. The flux / current method, inductance model method, intelligent control method and observer method all require the flux characteristics of the motor. However, the flux is a nonlinear function of the rotor position and phase current, which makes it very difficult to estimate the rotor position.

[0004] For medium and high speed operation, the phase current slope method is proposed to detect the characteristic position when the phase current slope is constantly changing. After the traditional current slope method detects the maximum current point, the motion back electromotive force will be significantly greater than the average voltage applied to the two ends of the winding, and the phase current will be significantly suppressed, which will lead to a reduction in the output torque of the motor. In order to improve the utilization rate of the DC bus voltage and increase the output torque, the present invention proposes a current slope position sensorless torque boost control method that combines voltage chopping control with current chopping control. Summary of the invention

[0005] The main purpose of the present invention is:

[0006] To overcome the problem of low output torque of traditional switched reluctance motor position sensorless control based on current slope, a simple and easy to implement sensorless torque boost control method for switched reluctance motor in medium and low speed range is proposed, which does not require any prior knowledge of the electromagnetic characteristic data and accurate mathematical model of the switched reluctance motor.

[0007] In order to achieve the above object, the present invention adopts the following technical scheme:

[0008] (1) Improve the output torque of the switched reluctance motor;

[0009] At the beginning of the excitation cycle, the system operates in voltage chopping control mode. When the motion back EMF is greater than the voltage applied across the winding, the phase current starts to decrease and the current maximum point can be detected. After that, the controller no longer needs information about the phase current, so it can switch to current chopping control mode, which is a current hysteresis control. As long as the available DC bus voltage is greater than the generated motion back EMF, the current can be adjusted to the required current reference value, thereby increasing the output torque of the motor.

[0010] (2) Calculation of current slope;

[0011] Before the control mode in step (1) switches to current chopping control, the current of each phase winding of the motor gradually increases, and the current of each phase winding is sampled in real time by the current sensor. The slope of the current of each phase winding can be obtained by calculating the difference between the sampled currents at two moments before and after in the DSP controller.

[0012] (3) Detection of special locations;

[0013] The phase current slope calculated in step (2) is filtered, and the zero crossing point of the phase current slope in each cycle is detected in real time in the controller, and a position detection pulse is issued. In each cycle of the motor, a position retrieval pulse is obtained, that is, the position where the stator and rotor teeth of the motor start to align.

[0014] (4) Rotor position estimation;

[0015] The position retrieval pulse obtained in step (3) estimates the rotor speed and rotor position. The phase difference between the position retrieval pulses emitted at the two adjacent current maximum positions is a fixed electrical cycle of 45 degrees. Therefore, the real-time speed of the motor can be calculated and the rotor position information can be estimated by the rotor position pulse signal obtained at the current maximum position.

[0016] Compared with the existing position-free algorithm of the switched reluctance motor, the present invention has the following beneficial effects:

[0017] 1. After detecting the special position point of the maximum current, the present invention switches the control mode from voltage chopping control to current chopping control to improve the output torque of the motor. The maximum position point of the phase current is detected by programming the DSP controller. The special position point is where the stator and rotor begin to align. The motor speed is calculated through the search pulses emitted at the two adjacent maximum current positions, the rotor position information is estimated, and the motor output torque can be improved.

[0018] 2. The present invention does not need to predict the electromagnetic characteristic data and motor body parameters of the motor, does not need to add additional hardware resources, is simple to control, easy to implement, and can significantly improve the output torque of the motor. Therefore, it has strong practicality and can adapt to operating environments with extreme conditions such as high temperature, high speed, high pollution, and strong electromagnetic interference. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 It is a schematic diagram of the control principle of the switched reluctance motor combination;

[0020] Figure 2 It is a schematic diagram of the principle of the current slope method under voltage chopping control;

[0021] Figure 3 It is a schematic diagram of the relationship between the relative position of the stator and rotor and the phase inductance;

[0022] Figure 4 It is a schematic diagram of the principle of torque boosting of a switched reluctance motor without a position sensor;

[0023] Figure 5 It is the block diagram of the switched reluctance motor speed control system; DETAILED DESCRIPTION

[0024] The technical solution of the present invention is described in detail below in conjunction with the accompanying drawings.

[0025] The present invention proposes Figure 1 The current slope position sensorless torque boost control method combining voltage chopping control and current chopping control is shown, which detects the maximum position point of the phase current in each cycle, that is, the position where the stator and rotor teeth of the switched reluctance motor begin to align. The rotor position pulse signal obtained from the maximum current position point is used to calculate the real-time speed of the motor, estimate the rotor position information, and improve the output torque of the motor, which has strong versatility and practicality.

[0026] Figure 2It is a schematic diagram of a switched reluctance motor using a current slope method to detect rotor position information in a voltage chopping control mode. In the voltage chopping control mode, when the conduction phase is turned off, the phase current enters a freewheeling mode, and when the current slope is detected to pass through the zero point, a search pulse signal is sent to calculate the real-time speed of the motor and estimate the rotor position. After the current slope is detected to pass through the zero point, the motion back electromotive force generated will be significantly greater than the voltage applied to both ends of the winding, and the current will drop, resulting in a reduction in output torque. In response to the above problems, the present invention proposes a method that combines voltage chopping control with current chopping control to improve the output torque of the motor under the control of the current slope method.

[0027] The relationship curve between the relative position of the stator and rotor of the SRM and the phase inductance is as follows: Figure 3 The output torque of SRM can be expressed by formula (1):

[0028]

[0029] In the formula, T is the output torque, i is the current flowing through the phase winding, L is the phase inductance, and θ is the rotor position. It can be seen from formula (1) that the output torque is a function of the current, and the inductance changes with the change of the rotor position. This means that each phase must be turned on in the inductance rising area to generate positive torque.

[0030] Assuming that the motor phase inductance is a linear function of the rotor position, the phase voltage equation of the SRM is:

[0031]

[0032] Where U dc is the terminal voltage of the phase winding, R is the resistance of the phase winding, and i is the current flowing through the phase winding. From equation (1), it can be seen that the voltage across the phase winding is equal to the sum of the three voltages. The first term on the right side of the equation is the winding resistance voltage drop; the second term is the transformer potential; and the third term is the motion back electromotive force.

[0033] According to the phase voltage equation of SRM, ignoring the influence of mutual inductance, the current gradient is:

[0034]

[0035] Depend on Figure 3 From the relationship curve between the relative position of the stator and the rotor and the phase inductance, it can be seen that when the SRM is running electrically, the main switch device is turned on in the range of θ1 to θ2, that is, θ1<θ on <θ2; turn off in the range of θ2 to θ3, that is, θ2<θ off <θ3. In the voltage chopping control mode, the phase current, phase inductance and position angle waveforms within one inductance change cycle are as follows: Figure 2 shown.

[0036] Figure 4 The schematic diagram of the torque boost principle of the switched reluctance motor without position sensor. Since no more information about the phase current is needed after the maximum current point is detected, the controller can switch to the current chopping control mode, which is a hysteresis current control. As long as the available DC bus voltage is greater than the generated motion back EMF, the phase current can be adjusted to the required current command, thereby improving the output torque of the motor.

[0037] In each cycle of the motor, a special rotor position reference point corresponding to the maximum phase current can be obtained. The time difference between two adjacent special position reference points is calculated, and the real-time speed of the motor is obtained according to formula (4):

[0038]

[0039] Where Δθ and Δt are the angle difference and time difference between position retrieval pulses in adjacent electrical cycles, respectively.

[0040] According to the special position where the stator and rotor teeth of the motor begin to align and the real-time speed, the arbitrary position of the rotor is estimated by equation (5):

[0041] θ(k+1)=θ(k)+ωΔT (5)

[0042] Where θ(k+1) and θ(k) are the position angle values ​​of adjacent sampling periods respectively; ΔT is the DSP interrupt period.

[0043] Figure 5 The figure shows the block diagram of the SRM speed control system. The switched reluctance motor speed control system is mainly composed of the SR motor body, power converter, controller, current detection circuit and position detection circuit. The controller is the core of the system. The current detection circuit collects the current signal, which is sent to the DSP controller after being processed by the current conditioning circuit. The detection of the maximum current position point and the position sensorless control algorithm in the present invention are both realized through DSP software programming. The digital controller gives the driving signal of the power converter according to the position information of the rotor to drive the motor to run and the PWM duty cycle is adjustable.

Claims

1. A method for controlling torque boost of a switched reluctance motor without a position sensor, characterized in that: The steps include: (1) Improve the output torque of the switched reluctance motor; At the beginning of the excitation cycle, the system operates in the voltage chopping control mode. When the motion back EMF is greater than the voltage applied to the two ends of the winding, the phase current begins to decrease and the maximum current point can be detected. After that, the controller no longer needs the information of the phase current, so it switches to the current chopping control mode, which is a current hysteresis control. As long as the available DC bus voltage is greater than the generated motion back EMF, the current can be adjusted to the required current reference value I ref , thereby increasing the output torque of the motor; (2) Calculation of current slope; The system operates in the voltage chopping control mode. The current of each phase winding of the motor gradually increases. The current sensor samples the current of each phase winding in real time. The DSP controller calculates the difference between the currents sampled at the previous and next moments to obtain the slope of the current of each phase winding. (3) Detection of special locations; The phase current slope calculated in step (2) is filtered, and the zero-crossing point of the phase current slope in each cycle is detected in real time in the controller, and a position search pulse is issued. In each cycle of the motor, a position search pulse is obtained, that is, the position where the stator and rotor teeth of the motor start to align; (4) Rotor position estimation; The position retrieval pulse obtained in step (3) estimates the rotor speed and rotor position. The phase difference between the position retrieval pulses emitted at two adjacent current maximum positions is a fixed electrical cycle of 45 degrees. Therefore, the real-time speed of the motor can be calculated through the rotor position pulse signal obtained at the current maximum position, and the rotor position information can be estimated.

Images